When Miguel Luna was an 8-year-old in the city of Cúcuta, Colombia, his family sometimes went days without water. The municipality would just shut it off, he recalls. "Nothing would come out of the faucets." When the water returned, his grandmother, Hercilia, would ceremoniously drink a glass before bedtime. "She'd say to us, 'Water is the most important thing in the world. We cannot live without it. We have to appreciate it, to protect it.' "

And to do that, he adds, "We have to understand where it comes from."

Now 40, Luna, who immigrated to the U.S. as a teenager, has spent much of the last decade trying to etch his grandmother's words into the youthful minds of urban Los Angeles County. He has brought teenagers to the banks of trash-choked city streams, taught them how to collect water samples and test for contaminants. He has led them to the sources of their own tap water: The rivers of the Sierra, the now-dusty valley that was once Owens Lake. He's shown them that, despite all the times they'd been told they lived in a desert, their hometown is crisscrossed with streams and creeks, a few of which still haven't been boxed up with concrete.

He has also taught them to become advocates for clean-running tap water, and for local watering holes that could become clean enough to wade in. And he has begun arming a select few –– those who emerge from his teaching sufficiently engaged and willing –– with the skills they need to work on behalf of water, as more than just volunteers.

"I started to see that we could go an extra step and say, 'How do we take this information and put you on track to get a job in this field?' " Luna says. "Because it's a growing field." Like much of Southern California, Los Angeles County and many of its 88 cities are in a mad rush to meet stringent new limits on coastal pollution in the next few years, instigating dozens of projects to contain and clean up urban runoff and storm water: soccer fields built over rainwater-capturing cisterns; engineered swales that let water percolate back into the aquifer.

"All of these projects need people to monitor whether they're doing well, whether they're meeting goals of capacity and quality," Luna says. He was already teaching young people to monitor local water quality. Why not have them monitor these projects for a living?

Goateed and genial, his black hair just beginning to gray, the ever-smiling Luna has the presence of a leader without threatening authority; troubled young people averse to taskmasters cotton to him. "He has charisma," says one of his former recruits, 20-year-old Cesar Villareal. "You want to be a part of what he's doing." Luna got his start in 2002 at the nonprofit group Heal the Bay, where he learned the science of collecting and testing water. Heal the Bay was founded in 1985 to make the polluted Santa Monica Bay safer for surfers and swimmers. But it soon became clear that you couldn't clean up the bay just by picking up cigarette butts, or even fixing the local sewage plant; you had to travel up the Los Angeles River and deal with its tributaries. And that required reaching out to the people in those upstream communities, many of whom spoke only Spanish and lived in near-poverty.

Luna joined Heal the Bay as a liaison to those Spanish-speaking residents, and soon began to see the value of repairing urban waterways not simply for the sake of the beach, but for the health and welfare of park-poor and flood-prone inland communities. In 2005, he started his own nonprofit, Urban Semillas, to create a cadre of water activists who would show up at meetings held by the county and state water boards. He spent his small budget on stipends instead of flyers. "Sometimes a community member would come to participate in a forum where everyone else was getting paid," he says. "So why shouldn't we pay the person with the most valuable information?"

Luna's passion, however, remained working with young people. "They're not set in their ways," he says. "You can change their minds." So out of Urban Semillas came Agua University, a three-month program in water education for high school students. "Agua University was a mechanism to engage," he says. "Everything we created had to be interactive -- it had to pass the 'cool' test." The curriculum also had to be accessible to teens with limited science education. "We've never used the word watershed," Luna says. "But at the end of the course everyone understood what a watershed was. We've never referred to watershed management, but everyone understood what it meant that there were stressors on the watershed, and they had to be reduced."

Luna brought his students to water sources and encouraged them to ask questions. Why does Malibu Creek, which carves a deep canyon through the still-wild Santa Monica Mountains, look different from Compton Creek, which trickles through a dense urban area? What kind of wildlife returns after the concrete is jackhammered out of a creek like Arroyo Seco? When his students tested water for nitrogen or heavy metals, he'd ask them to find the source: "Is there a nursery nearby? A factory?"